1. Field of the Invention
The present invention relates to a spinning method for forming, at a side wall portion of a cylindrical workpiece, a concave portion dented radially inwardly of the cylindrical workpiece.
2. Related Art
Conventionally, as a wheel to which a tire for a vehicle such as an automobile is fitted, a two-piece wheel has been widely used. The two-piece wheel is provided in the following manner. A wheel rim (hereinafter also simply referred to as a “rim”) substantially cylindrically formed from a plate-like material is combined with a wheel disc (hereinafter also simply referred to as a “disc”) formed into a substantially disc-like shape, and the wheel rim and the wheel disc are connected to each other by welding, thus providing the two-piece wheel.
Of these two components, the rim is formed by reducing a diameter of an approximate center of the cylindrical workpiece, and by forming an outer surface thereof into a concave cross-sectional shape. The rim is fabricated in the following manner, for example. First, end faces of a rectangular plate material are butted against each other to provide a cylindrical workpiece (cylindrical body). The butted end faces are connected to each other by resistance welding, friction stir welding or the like. Next, this cylindrical workpiece is held at a die provided with a predetermined concave portion, and then spinning is performed on a side wall portion of the workpiece In this spinning method, a roller is pressed to the workpiece from its outer surface toward its inside while the workpiece is rotated, thereby reducing a diameter of the workpiece. Thus, a concave portion, which is referred to as a “drop portion”, is formed in the side wall portion so as to be dented along a circumferential direction. In some cases, an end portion of the workpiece is bent to provide a curled portion.
A wheel can be obtained by inserting the foregoing disc into the rim fabricated in the above-described manner, and by connecting the disc and the rim by MIG welding and/or spot welding.
In the foregoing spinning method, ends of a workpiece are fixed to a die by a clamp and/or a plate, for example, so that the relative position of the workpiece, which is being machined, and the die will not be deviated. On the other hand, upon formation of a concave reduced-diameter portion in an approximate center of a workpiece by spinning, extension is caused in the workpiece along an axial direction. Therefore, JP-A-2000-288669 proposes a method in which while ends of a workpiece are fixed by a pair of plates, the distance between these plates is increased in a manner that follows extension caused during machining of the workpiece. In the method proposed in JP-A-2000-288669, the workpiece is sandwiched between the pair of plates, and in this state, a side wall portion of the workpiece is pressed by a roller. Then, with this pressing, a thickness of the cylindrical body, which has been pressed, is drawn. In other words, so-called “ironing” proceeds.
When end portions of a workpiece are clamped as described in JP-A-2000-288669, a plurality of clamp mechanisms are required in order to clamp each end portion. Therefore, a forming apparatus is complicated in structure, and in addition, capital investment is increased.
Further, in this method, positions of the plates have to be precisely controlled, which might result in a large-scale apparatus.
Besides, since forming cannot be performed on a clamped region, the degree of flexibility with respect to forming is low. Hence, it is difficult to obtain a formed article having a complicated shape.
Another example of spinning is described in JP-A-2004-314117.
Furthermore, in conventional spinning, a thin thickness portion disadvantageously occurs in a workpiece.
A change in workpiece plate thickness when a cylindrical workpiece is reduced in diameter will be described. FIG. 23 is a cross-sectional view illustrating structures of a workpiece Wu before machining and a workpiece Wp after machining. More specifically, FIG. 23 is a diagram illustrating an example in which spinning is performed on the cylindrical workpiece Wu, thereby forming the product Wp having a concave cross-sectional shape. In the spinning, while a roller R is pressed to the workpiece Wu in a radial direction with the workpiece Wu rotated around an axis parallel to an axial direction, the roller R is moved along the axial direction, thereby forming the product Wp.
Upon carrying out of the above-described spinning on the workpiece, each element of the workpiece is extended along the axial direction and is shrunk in the radial direction. Basically, the plate thickness of the workpiece is reduced upon extension of the workpiece along the axial direction, but the plate thickness of the workpiece is increased upon shrinkage of the workpiece in the radial direction because its circumferential length is shortened.
As described above, in accordance with the resulting cross-sectional shape, a reduction in diameter of the cylindrical workpiece causes: a portion in which the plate thickness is increased as a result of redundancy of material due to shrinkage of the workpiece in the radial direction; and a portion in which the plate thickness is reduced as a result of deficiency of material due to extension of the workpiece along the axial direction. For example, when a concave portion, having a narrow and deep cross-sectional shape as illustrated in FIG. 23, is formed in the workpiece, the plate thickness of a portion with the smallest diameter, indicated by the broken line in FIG. 23, tends to be reduced. Furthermore, in this portion, a region with a large curvature, indicated by alternate long and short dashed lines in FIG. 23, tends to be particularly reduced in plate thickness.
As described above, upon occurrence of a thin thickness portion in a product, the strength of the entire product is reduced. Therefore, in order to ensure a sufficient strength of a product, a thick workpiece has to be used in anticipation of a reduction in plate thickness, thus making it difficult to reduce product weight.